The present invention relates to a thin film transistor having a transparent semiconductor film and to a matrix display device using the same.
A matrix display device such as a liquid crystal display device includes: picture elements disposed in a matrix manner; and switching elements each of which applies a driving voltage to each picture element. When the switching element is turned ON, the driving voltage is applied to the picture element. When the switching element is turned OFF, the driving voltage is not applied to the picture element. A thin film transistor (TFT) which is widely used as such switching element is typically such that amorphous silicon is used to constitute a semiconductor layer.
Since the amorphous silicon exhibits the conductivity in accordance with irradiation of light, it is necessary to provide a channel-light-shielding film so as to prevent the deterioration of the switching characteristic. The provision of the shielding film causes the following problems: the manufacturing steps of the thin film transistor is increased, and decrease in an aperture ratio of the picture element causes a manufacturing cost to increase, and a display performance is deteriorated. Thus, in order to solve the problems, a transistor constituted of a transparent semiconductor material having no photocurrent is proposed.
For example, the transparent semiconductor material is disclosed in (Document 1) Japanese Unexamined Patent Publication No. 251705/1993 (Tokukaihei 5-251705)(Publication date: Sep. 28, 1993), (Document 2) Japanese Unexamined Patent Publication No. 067187/1994 (Tokukaihei 6-067187)(Mar. 11, 1994), and (Document 3) U.S. Pat. No. 5,744,864 (Date of Patent: Apr. 28, 1998).
The Document 1 discloses a technique such that: by using a transparent semiconductor layer whose carrier concentration is not more than 1018 cmxe2x88x923 when an energy band gap is not less than 3 eV, the shielding film is not required, so that the aperture ratio is increased. Further, the Document 2 discloses a technique such that: in order to increase the aperture ratio and decrease the manufacturing steps, one transparent semiconductor film constitutes a source section, a channel section, and a drain section of a liquid crystal driving transistor, and a liquid crystal driving electrode. Further, the Document 3 discloses an example in which, in order to obtain a transparent switching element, a degenerate semiconductor material having a band gap of not less than 2.5 eV as a channel layer is used.
Here, FIG. 10 shows a Vg-Id characteristic of a conventional thin film transistor. The thin film transistor is such that: an SiNx (single layer), formed in accordance with a PECVD method typically used in manufacturing an a-Si (amorphous silicon) TFT, is used as a gate insulating film, and ZnO is used as a semiconductor material. FIG. 10 shows that: in a TFT whose size is suitable for a typical liquid crystal display having a channel size of approximately L/W=5/15 xcexcm (L is a channel length, W is a channel width), it is possible to obtain the same effect as an a-SiTFT etc. whose field effect mobility (xcexcFE) is 0.52 cm2/Vs.
Further, a technique for obtaining a high-quality semiconductor material is disclosed in (Document 4) Japanese Unexamined Patent Publication No. 59087/1997 (Tokukaihei 9-59087)(Publication date: Mar. 4, 1997) and (Document 5) EP 1172858 A1 (Publication date: Jan. 16, 2002).
The Document 4 discloses a forming method of a thin film which includes the steps of: providing an intermediate layer, made of material different from a film forming material, on a surface of a glass substrate; and providing the film forming material on the intermediate layer, so as to improve the orientation of the film forming material. Further, the Document 5 discloses a forming method of a high-quality semiconductor thin film which is almost monocrystal. The semiconductor thin film is provided by using a material, in which the lattice misfit hardly occurs, as a base substrate.
In order to manufacture a higher-definition display or miniaturize a TFT so as to obtain a higher aperture ratio, it is necessary to further improve a TFT characteristic (mobility, ON/OFF ratio etc.).
In order to improve the TFT characteristic, it is necessary to improve a crystalline characteristic of a semiconductor material forming a channel, and to decrease the defective level in an interface between a semiconductor and a gate insulating film. Thus, it is important to properly select a material for an insulating film which constitutes the interface in combination with the semiconductor material.
However, according to the arrangements of the transistors etc. recited in the respective documents disclosing the transparent semiconductor material, it is not considered at all that: the insulating film exerts influence on the crystalline characteristic of the transparent semiconductor material, and various influence is exerted on a condition of the interface and the transistor characteristic of the TFT that has been manufactured. Further, the gate insulating films that constitute the channel and the interface respectively are made of a single insulating material. In a case where the insulating film using the SiNx (single layer) and the semiconductor layer constitute the interface in particular, SiNx deprives oxygen of ZnO, so that a crystalline characteristic of ZnO in the periphery of the interface is deteriorated.
While, in the forming method of the high-quality thin film, a substrate material for forming a thin film in a sedimentary manner, an intermediate layer and a buffering layer formed between the substrate material and the thin film are mentioned.
However, the technique is a technique for forming a thin film so as to be almost monocrystal, and it is not such that: the intermediate layer and the buffering layer are used as the gate insulating film when the thin film is used as the semiconductor layer of the field-effect-type transistor
As described above, the prior art does not give any consideration with respect to the improvement of the TFT characteristic brought about by the selection of the gate insulating film in a case where the transparent semiconductor material is applied to the thin film transistor.
The object of the present invention is to provide (a) a thin film transistor having a gate insulating film which can improve the TFT characteristic, and (b) a matrix display device having the same, so as to apply the present invention to a switching element in a matrix display device, made of a transparent semiconductor material, more widely and more effectively.
In order to solve the foregoing problems, a thin film transistor of the present invention includes: a semiconductor layer made of ZnO, Mgx Zn1xe2x88x92x O, Cdx Zn1xe2x88x92x O or CdO or ZnO, Mgx Zn1xe2x88x92x O, Cdx Zn1xe2x88x92x O or CdO that have been doped with (a) an element that can be univalent or (b) Ni; and a gate insulating film, wherein the gate insulating film has: a first insulating film made of a material other than an oxide for constituting an interface in combination with a gate electrode; and a second insulating film, sandwiched between the first insulating film and the semiconductor layer, that has an oxide for constituting interfaces in combination with both the first insulating film and the semiconductor layer.
According to the arrangement, the oxide is used in the second insulating film, so that it is possible to keep the interface consistency under an extremely preferable condition. Further, a material other than the oxide is used in the first insulating film, so that it is possible to further improve an insulating characteristic of the thin film transistor manufactured in a relatively low temperature range, at which an a-SiTFT process is performed, compared with the second insulating film including the oxide.
In this manner, the gate insulating film is constituted of two insulating films different from each other, so that it is possible to improve a crystalline characteristic of the semiconductor layer that constitutes the interface in combination with the second insulating film, and it is possible to decrease the defective level of the interface between the semiconductor layer and the second insulating film. Further, the second insulating film is constituted of the oxide, so that it is possible to restrain a material for the second insulating film from depriving oxygen of the semiconductor layer (ZnO etc.). Therefore, a crystalline characteristic of the semiconductor layer is kept under a preferable condition in the vicinity of the interface between the second insulating film and the semiconductor layer. As a result, it is possible to realize a thin film transistor having such a preferable switching characteristic that: a leak current level is low in an OFF area, and a mobility is high. Thus, it is possible to readily realize a thin film transistor having a transparent semiconductor film.
In order to solve the foregoing problems, another thin film transistor of the present invention includes: a semiconductor layer made of ZnO, Mgx Zn1xe2x88x92x O, Cdx Zn1xe2x88x92x O or CdO or ZnO, Mgx Zn1xe2x88x92x O, Cdx Zn1xe2x88x92x O or CdO that have been doped with (a) an element that can be univalent or (b) Ni; and a gate insulating film, wherein the gate insulating film has: a first insulating film made of a material other than an oxide for constituting an interface in combination with a gate electrode; and a second insulating film, sandwiched between the first insulating film and the semiconductor layer, that has an oxide for constituting interfaces in combination with both the first insulating film and the semiconductor layer, and the second insulating film is made of KNbO3, KTaO3, BaTiO3, CaSO3, CaZrO3, CdSnO3, SrHfO3, SrSnO3, SrTiO3, YScO3, CaHfO3, MgCeO3, SrCeO3, BaCeO3, SrZrO3, BaZrO3, LiGaO2, mixed crystal (Li1xe2x88x92(x+y)NaxKy)(Ga1xe2x88x92zAlz)O2 of LiGaO2, or a solid solution including at least two of these oxides.
According to the arrangement, the oxide is used in the second insulating film, so that it is possible to keep the interface consistency under an extremely preferable condition. Particularly, as to the oxide and the solid solution, interstitial inconsistency is little with respect to ZnO etc. for constituting the second insulating film, so that it is possible to keep the interface consistency under an extremely preferable condition. Further, a material other than the oxide in the first insulating film is used, so that it is possible to further improve an insulating characteristic of the thin film transistor manufactured in a relatively low temperature range, at which an a-SiTFT process is performed, compared with the second insulating film using the oxide.
In this manner, the gate insulating film is constituted of two insulating films different from each other, so that it is possible to improve a crystalline characteristic of the semiconductor layer that constitutes the interface in combination with the second insulating film, and it is possible to decrease the defective level of the interface between the semiconductor layer and the second insulating film. Further, the second insulating film is constituted of the oxide, so that it is possible to restrain a material for the second insulating film from depriving oxygen of the semiconductor layer (ZnO etc.). Therefore, a crystalline characteristic of the semiconductor layer is kept under a preferable condition in the vicinity of the interface between the second insulating film and the semiconductor layer. As a result, it is possible to realize a thin film transistor having such a preferable switching characteristic that: a leak current level is low in an OFF area, and a mobility is high. Thus, it is possible to readily realize a thin film transistor having a transparent semiconductor film.
In the thin film transistor formed in a staggered manner, in a case where a light-shielding film is not required, there are formed a source electrode, a drain electrode, and a semiconductor layer on the substrate. Thus, some materials for the substrate have unfavorable lattice misfit with respect to a semiconductor, and this deteriorates a characteristic of the thin film transistor. Then, the semiconductor layer is formed on the base layer having the oxides or the solid solution thereof, so that the lattice consistency with respect to the semiconductor layer becomes preferable as in the second insulating film of the thin film transistor. Thus, the characteristic of the semiconductor layer is restrained from deteriorating regardless of the substrate material, so that it is not necessary to limit the substrate material to a material having the preferable lattice consistency with respect to the semiconductor layer.
In order to solve the foregoing problems, a matrix display device of the present invention includes switching elements disposed in a matrix manner, and is arranged so that any one of the foregoing thin film transistors is used as each of the switching elements.
By using the thin film transistor having high performance, a switching characteristic in the matrix display device is improved.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.